The invention relates to a static converter comprising a filter protecting against high-frequency disturbances, the static converter comprising an external housing earthed by a protective conductor, input cables, connected respectively to the mains and to a battery bank, and an output cable connected to a user device.
The high-frequency disturbances due to switchings of the power semi-conductors, notably thyristors, in static converters can give rise to mains disturbances which have to be limited as far as possible.
To achieve this, insertion filters connected to the equipment frame are used in state-of-the-art devices. These filters are conventional low-pass LC filters, for example .pi.-structured, inserted in line at the converter input and output, between the input or output cables and the converter frame, itself connected to the earth of the building in which it is installed.
As soon as the power of the converter reaches several tens of KVA, these filters require high-value capacitors to be used. The currents fed back via these capacitors to the protective conductor connecting the equipment frame to earth then very quickly exceed the permissible values. Indeed, these currents must remain limited to low values of a few milliamperes, in order not to cause spurious tripping of the insulation monitoring devices, in particular the differential circuit breakers located on the line side of the converter. With high-value capacitors, however, the currents, notably derived from the mains supply, which are fed back to the protective conductor are seen by the insulation monitoring devices as a permanent fault. Moreover, increasing the tripping threshold of these monitoring devices too much would result in real earth faults being masked and in the protection of persons in contact with this equipment being inacceptably reduced.
Furthermore, filtering the real common mode is very difficult to achieve with state-of-the-art in-line insertion devices and it is difficult to conciliate interference limiting requirements, i.e. a high-value inductance in the conventional LC filter, with the small dimensioning requirements due to the nominal current the inductance has to withstand.